Arcyd: fix potential missing method exception

abdcmd_processrepos.RefreshCachesOperation is expecting
abdt_arcydreporter to have a method 'on_tryloop_exception' so we need to
actually provide it.

Test Plan: $ ./precommit.sh
Arcyd: support batch 'snooping' of info/refs

Spend less time doing 'git fetch' by providing a way to specify a
URL for 'info/refs' of each of the repositories.

These URLs can then be fetched in a batch together, re-using
connections where possible.

The contents of the URLs can be watched for changes, when they change
then the corresponding repo must be fetched.

See http://git-scm.com/book/en/Git-Internals-Transfer-Protocols for
more on 'info/refs'.

Add 'phlurl_request' for easy and efficient fetching of URLs from the same
host.

Add 'phlurl_watcher' for easy and efficient montoring of URLs.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
$ ./testbed/arcyd/loop_test.sh
TODO: UPDATE LOOP TEST WITH CACHE TEST